Dunkin Donuts is offering one free beverage if you sign up for a DDPerks account with promo code: AMEX
I got this offer via email as an AMEX member, but given that it's a generic promo code and you don't need to enter a credit card number, I'm pretty sure you don't need to be an AMEX member.
DDPerks Sign-up link:
Full Offer Details (via email):
Join DD Perks® with promo code AMEX and unlock your first free beverage, plus get access to member-only promotions and On-the-Go Mobile Ordering in the Dunkin' app!
PROMO CODE: AMEX